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Orderly Recruitment of Muscle Fibres

Muscle fibre activation

Motor Units

Sequential firing depending on load and repetitions

Taking the mystery out of muscle fibre contraction

  • The following graphic and text will take the mystery out of muscle fibre contraction.
  • Training for endurance, strength or hypertrophy [size] is all a matter of targeting the right muscle fibre using the right loading, sets and repetitions.
  • The load and the number of repetitions to exhaustion will determine which motor units will be exercised the most.

What is a motor unit?

  • A motor unit is the number of muscle fibres triggered by the firing of a single alpha motor neurone unit in the anterior horn of the spinal cord.
  • Each motor unit has the same type of muscle fibre - either Type-1's, the various intermediate fibre types,  Type-IIA's or Type-IIB's.
  • Slow-twitch [Type-1's] usually have small numbers of muscle fibres - approximately 100 fibres per unit17.
  • A fast-twitch motor unit  [Type-IIBs] may have approximately 10,000 fibres17.
  • Each muscle in the average human has about a 50:50 ratio of slow and fast twitch muscle fibres.
  • When an impulse travels down the nerve to its motor unit, all the fibres within that unit whether it is a Type-1, Intermediate or Type-IIB motor unit ---  will contract simultaneously.  This is the law of "all or none" in muscle physiology17.

What is the orderly [sequential] recruitment of muscle fibres?

  • With the right load, ie one that is modest enough to allow you to reach exhaustion after all the motor units are called into play will result in the weakest endurance Type-1 fibres firing first and then when these fibres are exhausted, the intermediate fibre motor units will fire and towards the end of the set, when your muscle is becoming exhausted and you can barely lift this load, the Type-IIB's which are the strongest fibres will contract.
  • The Type-1's are the weakest fibres but can contract for long periods of time if the load is light enough.
  • The Type-IIB muscle fibres are the strongest fibres but exhaust easily - however, these muscle fibres are the 'Holy Grail' of the bodybuilders because these fibres have the greatest potential to grow.

What about loads and muscle fibre recruitment?

  • Power Training [1- 3 reps to exhaustion] If the load is high, for example a 1-3 RM where you reach exhaustion in 1 to 3 repetitions, this load is heavy enough to simultaneously fire all muscle types with the Type-1's exhausting rapidly leaving only the strongest Type-IIA and mainly Type-IIB to finish the last repetition. This type of training is used for powerlifters where explosive but short-lived force is needed by the strongest muscle fibres. Myofibrillar hypertrophy occurs. Hypertrophy is due to true protein increase in the muscle due to satellite cells helping to increase the number and size of the contractile proteins [Actin and Myosin]. The number of muscle fibres stays the same in humans but the satellite cells fuse to existing cells and donate their nuclei and DNA to the existing fibres helping them to increase in size. To find out more click on 'The Mystery of Skeletal Muscle Hypertrophy'
  • Strength Training [2 - 6 reps to exhaustion] - this load will once again exhaust the Type-1 motor units rapidly leaving only the higher intermediates and Type-IIBs to complete the repetitions. Strength training is for those who want strength but also anaerobic endurance where resistance is being met relatively frequently such as football. Myofibrillar hypertrophy occurs due to satellite cells increasing the contractile proteins actin and myosin [see above]
  • Hypertrophy Training [8 - 20 reps to exhaustion] -This load and the number of repetitions will ensure an orderly firing of the Type-1, intermediates and Type-IIB motor units. Hypertrophy occurs significantly in Type-1 fibres due to resistance and aerobic training40 but more hypertrophy is possible  with Type-IIB fibres which have the greatest potential to grow. Unlike power and strength training, hypertrophy is not due to myofibrillar hypertrophy but due to sarcoplasmic hypertrophy where the sarcoplasma volume increases. The number of reps and the load used to enable this number of repetitions to be done will ensure all fibre types have the potential to grow.
  • Endurance Training [20 or more repetitions] - Type-1 fibres are endurance fibres and recover rapidly compared with fast-twitch fibres. Thse Type-1 fibres are ready to contract again in 90 seconds. Ideally with endurance training, a weight should be used that will allow repetitive lifting or pushing of that load without exhaustion occuring within 90 seconds. The reason for this is that if a light enough load is used theoretically you can train just the Type-1 muscle fibres without progressing to the intermediates or fast-twitch fibres. Importantly, those that want to progress to hypertrophy should ensure that they never lift or push a load beyond 90 seconds to reach exhaustion as this will allow the Type-1 fibres to recover and 'kick-in' once again giving you a 'second wind'17. This will mean you will not trigger off the stronger intermediate and fast-twitch muscle fibres.

One set to failure

  • It has been dogma that many sets need to be done to elicit strength or hypertrophy.
  • A study out of Adelphi University looked a a large number of research papers on single vs multiple-set resistance training. Their conclusion:
    "Perhaps the most controversial element of any strength training programme is the number of sets required to increase muscular strength and hypertrophy. There is a prevalent belief that at least 3 sets of each exercise are required to elicit optimal increases in strength and hypertrophy. However, most of the studies that reported the results of training with single versus multiple sets do not substantiate this tenet. In fact, the preponderance of evidence suggests that for training durations of 4 to 25 weeks there is no significant difference in the increase in strength or hypertrophy as a result of training with single versus multiple sets42."
  • Dr Doug McGuff and John Little in their book 'Body by Science' also state that one set to failure is all that is required. They stress the point that once exhaustion is reached and the Type-IIB motor units have been engaged, it becomes redundant to perform any more repetitions as you have worked your way through the orderly recruitment of all the fibres to these top-level fast-twitch fibres. If you do one set well with the right load and perfect form, you will have reached your 'Holy Grail' if you want strength and hypertrophy. They also point out that it will take 4 - 10 days for the Type-IIB muscle fibres to recuperate and that in any given week, it would once again be pointless to exercise that particular muscle. It would be far better to perform split routines exercising various muscle groups, taking them to exhaustion in one set and then resting that muscle group for one week. Remember that famous aphorism that you grow out of the gym and not in the gym. Also to grow you need to take advantage of the all important immediate post-exercise metabolic window.

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